The present invention relates generally to headboxes used for distributing a paper pulp slurry onto a moving web in connection with a paper making process, and more particularly to an improved headbox having better slurry handling characteristics.
Older type headboxes which are either open to atmospheric pressure or which have a pressurized air cap utilize rotating perforated cylinders to stir and disperse pulp slurry within the headbox. These devices have several shortcomings, including relatively poor agitation and pulp dispersion, and buildup of pulp around the edges of the perforations. Furthermore, sometimes the perforations impart a memory on slurry flows, wherein ridges or streaks are formed in the outgoing slurry flow.
In modern hydraulic headboxes, slurry passes through a section having many channels wherein the cross-section of each channel varies along the direction of flow. The passages and channels of the headbox must be carefully designed and sized to generate a particular flow velocity and to generate microturbulence. The generation of microturbulence is necessary as this is the mechanism which prevents floc formation in the slurry. These headboxes, if not carefully sized or manufactured, can cause fiber roping and buildup at passage inlets and may fail to generate sufficient turbulence for pulp dispersion. They also can be quite sensitive to flow pulsations in the slurry delivery system.
It would be desireable to provide an improved headbox arrangement which provides for efficient agitation and dispersion of paper slurry in the headbox while avoiding the limitations of the prior art.
The present invention involves the use of efficient fluid dynamical stirring and dispersing means in a headbox instead of rotating perforated cylinders or a multitude of flow channels.
The fluid dynamical means set forth herein is in the form of Coanda nozzles placed in the headbox upstream of the slice. Coanda nozzles produce high shear, mixing, recirculation, and dilution. The "Coanda effect" which these nozzles create is exemplified by U.S. Pat. No. 2,052,869, issued to Henri Coanda. Briefly, this phenomenon can be described as the tendency of a fluid, which emerges from a slit under pressure, to attach itself or cling to and follow a surface in the form of an extended lip of the slit, which recedes from the flow axis of the fluid as it emerges from the slit. This creates a zone of reduced pressure in the area of the slit so that any entrainable material which is in the zone will become entrained and flow with the fluid which has attached itself to the extended lip.
U.S. Pat. No. 3,853,695 also describes the use of the Coanda effect as applied to headbox nozzles. There the Coanda nozzle is used immediately before the discharge slice to supply to the slurry "all the energy" needed to accelerate the slurry flow through the discharge slice at a particular discharge velocity onto the web-forming surface. In the currently proposed scheme, however, the Coanda nozzles are used only for slurry mixing; hence, energy requirements are greatly reduced.